Math Is Fun Forum

Nico van Os

Guest

Sum problem with exponent factor and unknown n

Hi,

I'm trying to solve a math problem and could use some help. I need to find a way to calculate the following.

My formula is this:   a * (1+ ∑ b^n) = Sn

My knows are a, b and Sn. My unknown is n.

So for example:
a = 4
b = 1.07
Sn = 23
calculate n

answer n = 4

explained:
S4 = 4 * (1+ ∑ 1.07^n) = 23

Since ∑ 1.07^n = 1.07 + 1.07^2 + 1.07^3 + 1.07^4 = 1.07 + 1.145 + 1.225 + 1.311 = 4.7507

4* (1+4.7507) = 23.00

So how can I derive n for any set of variables?

Thanks!

Bob

Administrator

Registered: 2010-06-20

Posts: 10,643

Re: Sum problem with exponent factor and unknown n

hi Nico van Os

Welcome to the forum.

is a geometric series and there's a formula for that.

If you make b^n the subject of the equation (and it's not obvious what n is) then take logs to make n the subject.

eg.

Hope that helps,

Bob

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

Olinguito

Member

Registered: 2014-08-12

Posts: 649

Re: Sum problem with exponent factor and unknown n

Take logarithms.

Last edited by Olinguito (2014-09-17 20:24:02)

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Bassaricyon neblina

nicovanos

Member

Registered: 2014-09-17

Posts: 1

Re: Sum problem with exponent factor and unknown n

Hi guys,

Thank you so much for helping me. This solved my problem. :-))

Question: how do you produce these formula notations? I can't find any explanation.

Math is indeed fun. Some formula's are still like magic to me.

anonimnystefy

Real Member

From: Harlan's World

Registered: 2011-05-23

Posts: 16,049

Re: Sum problem with exponent factor and unknown n

You can get those neat looking formulas using LaTeX.

To use it, you need the math tags:

You can click any formula to see how it was written. Also, there are a lot of them here

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